The present invention relates to a control circuit of a load supplied by an A.C. voltage. The present invention more specifically applies to a load directly connected to a network supplying a high A.C. voltage, for example the mains (for example, 220 or 110 volts).
When a load is to be supplied by the A.C. electric network, there often is a problem of overcurrent upon power-on. The problem is particularly present in the case of a load having, upon power-on, a low impedance as compared to its steady-state impedance. This is the case, for example, for a filament-type lamp, where the filament is cold upon power-on, or for a motor which has no emf as it starts.
A first disadvantage of a so-called xe2x80x9ccoldxe2x80x9d start on a load having a low impedance as compared to its nominal impedance is that it reduces the load lifetime. For a lamp, this creates a thermal shock onto the filament, which limits the lifetime thereof. It can indeed be noted that a light bulb most often breaks down at the lighting. For a motor, the overcurrent at the starting with respect to the nominal current adversely affects the lifetime of the brushes.
Another disadvantage of an unprotected cold start for this type of loads is that it often causes a flicker phenomenon on the other loads which may be connected to the same electric line. This phenomenon is due to the current surge peaks, repeated at each halfwave of the A.C. voltage (frequency of 50 or 60 Hz) as long as the load impedance has not reached a sufficient value. Such a parasitic phenomenon appears, in particular, on already lighted lamps, at the starting of a motor of a domestic appliance connected on the same electric network.
Most often, in applications where such low power-on impedance loads are used, the first reason for which the load is desired to be associated with a control circuit is a need for adjusting the load power. For example, it can be a light intensity adjustment for an incandescent lamp or an adjustment of the rotation speed of a motor.
FIG. 1 shows a first conventional example of a power variator of a load 1 to be supplied in A.C. mode. The circuit of FIG. 1 presently is, for a power variation control, one of the best performance/cost compromises.
Load 1 is connected, in series with a bidirectional switch, most often a triac 2, between two terminals E1, E2, that receive an A.C. supply voltage Vac. Triac 2 generally is controlled in phase angle by means of a circuit 3 connected in parallel on triac 2, that is, between terminal E1 and midpoint B of the series connection of the triac with the load.
Circuit 3 is formed of a diac 4, connected between the gate of triac 2 and the midpoint A of a series connection of a variable resistive element 5 with a capacitor C. The series connection of element 5 with capacitor C is connected in parallel with triac 2. Variable resistive element 5 is, for example, formed of a resistor R connected in series with a potentiometer P.
The operation of a control circuit illustrated by FIG. 1 is well known. This operation is briefly recalled hereafter in relation with FIG. 2, which shows an example of timing diagram of voltage VL across load 1 in steady state. It should be noted that, if load 1 is purely resistive, the shape of voltage VL also corresponds, in steady state, to the shape of current IL through the load.
At the beginning of each halfwave of supply voltage Vac, triac 2 is blocked and voltage VL across load 1 is zero. It is assumed that resistor R has a very high value so that the impedance of load 1 is negligible with respect to this value. As the amplitude of the A.C. voltage increases, capacitor C charges through resistor R and potentiometer P. When voltage VAB across capacitor C reaches the threshold voltage of diac 4, said diac turns on and a current then flows through the gate of triac 2. At this time (t0, FIG. 2), the triac triggers and the voltage across load 1 becomes voltage Vac, neglecting the voltage drop in triac 2. At the end of each halfwave, triac 2 blocks by the disappearing of the current flowing therethrough, and the above-described operation repeats at the following halfwaves.
The phase angle power variation is obtained by having the value of the resistor of element 5 vary by a variation of potentiometer P. Indeed, the greater the value of potentiometer P, the longer capacitor C will take to have across its terminals a sufficient voltage to trigger diac 4, and the later time to will come with resect to the beginning of the halfwave.
In steady state, that is, once load 1 has reached its nominal impedance, the circuit such as shown in FIG. 1 operates properly in power variation.
However, in transient state, that is, either at the powering-on or in case of an increase of the power of the load by decrease of the resistance of potentiometer P, such a circuit has the disadvantage of creating strong current peaks in the load. Indeed, when triac 2 turns on, the load then sees the mains voltage and, what is more, under a high amplitude due to the phase angle modulation performed for the steady state power variation. If the load has a very small impedance, this then almost create a short-circuit on the mains.
U.S. Pat. No. 4,680,536 discloses a control circuit of the same type as the invention and based on the diagram of FIG. 1. With respect to the diagram of FIG. 1, the circuit of this document comprises a rectifying bridge, the input terminals of which are connected between the common terminal of the resistor and the potentiometer and the common terminal of the triac and the load. The bridge comprises, in a diagonal, a second resistor, a second capacitor and a Zener diode. Such a circuit has for object to limit the current peaks in the load at the lighting.
A drawback of such a circuit is that it necessitates a voltage limiting component (Zener diode) for limiting the voltage in the bridge diagonal (at the terminals of the second capacitor); in the absence of such a limitor, the voltage in diagonal could reach the peak value of the A.C. voltage. Another drawback is that the second capacitor is however submitted to a relatively high voltage (higher than the breakover voltage of the diac, so that the circuit necessitates two capacitors that must support a voltage higher than the breakover voltage of the diac.
Another disadvantage of the circuit of FIG. 1 and of the circuit of U.S. Pat. No. 4,680,536 is that potentiometer P has to be a high voltage potentiometer, which is relatively expensive.
FIG. 3 shows a second conventional embodiment of a control circuit of a triac 2, series-connected with a load 1 to be supplied by means of an A.C. voltage Vac. The circuit of FIG. 3 aims not only at enabling a power variation of load 1 in steady state but also at limiting the surge current upon power-on, that is, as long as the load impedance is small as compared to its steady state impedance. Such a circuit requires a control block 6 of the actual traic and a supply block 7. Indeed, block 6 is generally made in the from of an integrated circuit and thus needs to be supplied by a regulated voltage provided by block 7. Block 6 is connected in parallel with triac 2, and thus is connected between terminals E1 and B of the assembly. Block 7 is connected to supply terminals E1 and E2 and must include a reference connection to node B. An output of block 6 is connected to the gate of triac 2 to ensure its control while an output of block 7 provides a D.C. regulated voltage to block 6.
An assembly such as illustrated in FIG. 3 enables obtaining satisfactory results, both for the power variation and for the transient state current surge limitation
However, it has the disadvantage of having a particularly complex structure and a high cost
The present invention aims at providing a novel solution to solve the surge current problem at the starting of loads supplied by an A.C. voltage, this solution palliating at least one of the drawbacks of the known solutions.
The present invention aims, in particular, at providing a solution which enables not only a variation of the load power in steady state, but also a limitation of the surge current when the load has a small impedance during in a transient state.
The present invention also aims at providing a solution which is of particularly simple implementation and which has a low cost with respect to conventional integrated circuit solutions.
The invention also aims at reducing the number of components necessary for making the circuit and in particular to avoid the use of voltage limiting components as in U.S. Pat. No. 4,680,536.
The invention also aims at avoiding the use of two capacitors that must support a voltage higher than the breakover voltage of a bidirectional conduction element (diac) used for controlling a bidirectional switch (triac) controlled by its phase angle.
To achieve these objects, the present invention provides a control circuit of an A.C. load, of the type including a bidirectional switch controllable by phase angle, in series with the load between two terminals of application of the A.C. supply, and including, in parallel with the switch, a first resistive element, a first capacitor and an element, in series with said first resistive element and said first capacitor, and operating, in steady state, like a constant current source, the midpoint of the series connection of the first resistive element and of the first capacitor being connected, via a bidirectional conduction element automatically turned on when the voltage thereacross exceeds a predetermined threshold, to a control terminal of the switch.
According to an embodiment of the present invention, the element in series with the first resistive element and the first capacitor is formed of a rectifying bridge in the diagonal of which are associated, in parallel, a second capacitor and a second resistive element.
According to an embodiment of the present invention, the value of the second capacitor is large with respect to the value of the second capacitor.
According to an embodiment of the present invention, the first resistive element is a fixed resistor, the second resistive element being a variable resistor.
According to an embodiment of the present invention, the first resistive element is a variable resistor, the second resistive element being a fixed resistor.
According to an embodiment of the present invention, the bidirectional switch is a triac.
According to an embodiment of the present invention, the automatically triggered bidirectional conduction element is a diac.